Android - Room 사용 방법

// RecyclerView
implementation "androidx.recyclerview:recyclerview:1.0.0"

// Room
implementation "androidx.room:room-runtime:2.0.0"
kapt "androidx.room:room-compiler:2.0.0"
testImplementation "androidx.room:room-testing:2.0.0"

// ViewModel and LiveData
implementation "androidx.lifecycle:lifecycle-extensions:2.0.0"
kapt "androidx.lifecycle:lifecycle-compiler:2.0.0"

class Word {
    private val word: String

    constructor(word: String) {
        this.word = word
    }

    fun getWord() = this.word
}

@Entity(tableName = "word_table")
class Word {

    @PrimaryKey
    @NonNull
    @ColumnInfo(name = "word")
    private val word: String

    constructor(word: String) {
        this.word = word
    }

    fun getWord() = this.word
}

@Dao
interface WordDao {

    @Query("SELECT * from word_table ORDER BY word ASC")
    fun getAllwords(): List<Word>

    @Insert
    fun insert(word: Word)

    @Query("DELETE FROM word_table")
    fun deleteAll()
}

@Dao
interface WordDao {

    @Query("SELECT * from word_table ORDER BY word ASC")
    fun getAllwords(): LiveData<List<Word>>

    @Insert
    fun insert(word: Word)

    @Query("DELETE FROM word_table")
    fun deleteAll()
}

@Database(entities = [Word::class], version = 1)
abstract class WordRoomDatabase : RoomDatabase() {
    abstract fun wordDao(): WordDao
}

@Database(entities = [Word::class], version = 1)
abstract class WordRoomDatabase : RoomDatabase() {

    companion object {
        @Volatile private var INSTANCE: WordRoomDatabase? = null

        fun getDatabase(context: Context): WordRoomDatabase {
            if (INSTANCE == null) {
                synchronized(this) {
                    INSTANCE = Room.databaseBuilder(
                        context.applicationContext,
                        WordRoomDatabase::class.java, "word_database"
                    ).build()
                }
            }
            return INSTANCE!!
        }
    }

    abstract fun wordDao(): WordDao
}

class WordRepository {

    companion object {
        private class insertAsyncTask constructor(private val asyncTaskDao: WordDao) :
                AsyncTask<Word, Void, Void>() {
            override fun doInBackground(vararg params: Word): Void? {
                asyncTaskDao.insert(params[0])
                return null
            }
        }
    }

    private val wordDao: WordDao
    private val allWords: LiveData<List<Word>>

    constructor(app: Application) {
        val db = WordRoomDatabase.getDatabase(app)
        wordDao = db.wordDao()
        allWords = wordDao.getAllwords()
    }

    fun getAllWords(): LiveData<List<Word>> {
        return allWords
    }

    fun insert(word: Word) {
        insertAsyncTask(wordDao).execute(word)
    }

}

class WordViewModel : AndroidViewModel {
    private val repository: WordRepository
    private val allWords: LiveData<List<Word>>

    constructor(app: Application) : super(app) {
        repository = WordRepository(app)
        allWords = repository.getAllWords()
    }

    fun getAllWords(): LiveData<List<Word>> = allWords
    fun insert(word: Word) {
        repository.insert(word)
    }
}

val adapter = WordListAdapter(this)
recyclerview.adapter = adapter
recyclerview.layoutManager = LinearLayoutManager(this)
recyclerview.addItemDecoration(
    DividerItemDecoration(this, DividerItemDecoration.VERTICAL)
)

wordViewModel = ViewModelProviders.of(this).get(WordViewModel::class.java)
wordViewModel.getAllWords().observe(this, Observer<List<Word>> { words ->
    adapter.setWords(words)
})

class NewWordActivity : AppCompatActivity() {

    companion object {
        val EXTRA_REPLY = "com.example.android.wordlistsql.REPLY"
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_new_word)

        button_save.setOnClickListener {
            val replyIntent = Intent()
            if (TextUtils.isEmpty(edit_word.text)) {
                setResult(Activity.RESULT_CANCELED, replyIntent)
            } else {
                val word = edit_word.text.toString()
                replyIntent.putExtra(EXTRA_REPLY, word)
                setResult(Activity.RESULT_OK, replyIntent)
            }
            finish()
        }
    }
}

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    super.onActivityResult(requestCode, resultCode, data)

    if (requestCode == NEW_WORD_ACTIVITY_REQUEST_CODE && resultCode == RESULT_OK) {
        val word = Word(data!!.getStringExtra(NewWordActivity.EXTRA_REPLY))
        wordViewModel.insert(word)
    } else {
        Toast.makeText(applicationContext, "Word not saved because it is empty",
            Toast.LENGTH_LONG).show()
    }
}